As a follow-up to our recent video on generator maintenance (Servicing an Onan RV Generator), when we cleaned the spark arrestor and changed the oil, oil filter & air filter, today we’re servicing the cooling system.

We flush the cooling system and replace the anti-freeze and radiator pressure cap on our Onan 7.5 kilowatt QuietDiesel generator every two years. Every other time (every four years) we also replace the thermostat. Unfortunately, the thermostat can only be replaced if you have access to the top of the generator’s engine, so the generator must be on a slide-out to do it yourself. The rest of the job can be done without a generator slide.

We use regular automotive radiator flush, anti-freeze and replacement pressure cap, all of which can be purchased at most auto parts stores. We purchased the new thermostat and gasket directly from Onan. We also chose to use pure antifreeze and mix it 50/50 with de-ionized or distilled water, as opposed to buying the 50/50 pre-mixed coolant. It doesn’t matter which you choose. Regular tap water can be used if it is low in minerals, but for such an important application, we don’t mind spending an extra dollar or two on bottled water (although we do the flushing and rinsing of the system with regular tap water).

The two most common challenges with this task are getting all of the coolant drained out of the system, and then getting all of the air back out of it. Onan published a service bulletin ( Onan Coolant Service Bulletin 981-1049 ) to address this, in which they recommend using care when filling the system, by avoiding pouring water, flush or coolant into the vent or overflow tubes. While the whole process is a bit time-consuming, the method we demonstrate has been very successful for us, and we’ve never had a problem getting our genset back up and running without incident.


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